From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752032AbcGRMtK (ORCPT ); Mon, 18 Jul 2016 08:49:10 -0400 Received: from mail-lf0-f53.google.com ([209.85.215.53]:33071 "EHLO mail-lf0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751441AbcGRMtE (ORCPT ); Mon, 18 Jul 2016 08:49:04 -0400 MIME-Version: 1.0 In-Reply-To: <20160715160253.GH21816@e105550-lin.cambridge.arm.com> References: <1466615004-3503-1-git-send-email-morten.rasmussen@arm.com> <1466615004-3503-7-git-send-email-morten.rasmussen@arm.com> <578646A8.1070607@arm.com> <20160713163723.GC21816@e105550-lin.cambridge.arm.com> <20160714151518.GD21816@e105550-lin.cambridge.arm.com> <20160715114615.GG21816@e105550-lin.cambridge.arm.com> <20160715160253.GH21816@e105550-lin.cambridge.arm.com> From: Vincent Guittot Date: Mon, 18 Jul 2016 14:48:42 +0200 Message-ID: Subject: Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain To: Morten Rasmussen Cc: Dietmar Eggemann , Peter Zijlstra , "mingo@redhat.com" , Yuyang Du , mgalbraith@suse.de, linux-kernel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15 July 2016 at 18:02, Morten Rasmussen wrote: > On Fri, Jul 15, 2016 at 03:39:05PM +0200, Vincent Guittot wrote: >> On 15 July 2016 at 13:46, Morten Rasmussen wrote: >> > On Thu, Jul 14, 2016 at 04:15:20PM +0100, Morten Rasmussen wrote: >> >> On Thu, Jul 14, 2016 at 03:25:36PM +0200, Vincent Guittot wrote: >> >> > On 13 July 2016 at 18:37, Morten Rasmussen wrote: >> >> > > Also, for SMT max capacity is less than 1024 already. No? >> >> > >> >> > Yes, it is. I haven't looked in details but i think that we could use >> >> > a capacity of 1024 for SMT with changes that have been done on how to >> >> > evaluate if a sched_group is overloaded or not. >> >> >> >> Changing SMT is a bit more invasive that I had hoped for for this patch >> >> set. I will see if we can make it work with the current SMT capacities. >> >> >> >> > >> >> > > But we may be able to cater for this in wake_cap() somehow. I can have a >> >> > > look if Vincent doesn't like this patch. >> >> > >> >> > IMO, rd->max_cpu_capacity field doesn't seem to be required for now . >> >> >> >> No problem. I will try to get rid of it. I will drop the "arm:" patches >> >> as well as they would have to be extended to guarantee a max capacity of >> >> 1024 and we most likely will have to change it again when Juri's DT >> >> solution hopefully gets merged. >> > >> > I have had a closer look at wake_cap() again. Getting rid of >> > rd->max_cpu_capacity isn't as easy as I thought. >> > >> > The fundamental problem is that all we have in wake_cap() is the waking >> > cpu and previous cpu ids which isn't sufficient to determine whether we >> > have an asymmetric capacity system or not. A capacity <1024 can either a >> > little cpu or an SMT thread. We need a third piece of information, which >> > can be either the highest cpu capacity available in the cpu, or a >> > flag/variable/function telling us whether we are on an SMT system. >> > >> > I see the following solutions to the problem: >> > >> > 1. Have a system-wide max_cpu_capacity (as proposed in this patch) which >> > can let us detect SMT systems as max_cpu_capacity < 1024 implies SMT. >> > >> > 2. Change SMT thread capacity to 1024 so we implicitly know that max >> > capacity is always 1024. As said above, this is a very invasive change >> > as it would mean that we no longer distinguish between SMP and SMT. >> > smt_gain and SD_SHARE_CPUCAPACITY would no longer have any effect and >> > can be ripped out. I would prefer not create a dependency on such a >> > massive change. We can do the experiment afterwards if needed. >> > >> > 3. Detect SMT in wake_cap(). This requires access to the sched_domain >> > hierarchy as the SD_SHARE_CPUCAPACITY is the only way to detect SMT, >> > AFAIK, apart from looping through the capacities of all cpus in the >> > system basically computing max_cpu_capacity each time. >> > wake_cap() is currently called before rcu_read_lock() that gives us >> > access to the sched_domain hierarchy. I would have to postpone the >> > wake_cap() call to being inside the lock and introduce another lookup in >> > the sched_domain hierarchy which would be executed on every wake-up on >> > all systems. IMHO, that is a bit ugly. >> > >> > I don't really like any of the solutions, but of those three I would go >> > for the current solution (1) as it is very minimal both in the amount of >> > code touched/affected and overhead. We can kill it later if we have a >> > better one, no problem for me. >> >> I had solution 2 in mind. I haven't looked deeply the impact but I >> thought that the main remaining blocking point is in >> update_numa_stats where it use the fact that the capacity is less than >> 1024 vat SMT level to compute task_capacity and set has_free_capacity >> only if we have less than 1 task per core. >> smt_gain would not be used anymore > > Isn't group capacities of also smaller and hence influence load > balancing decisions? It should not because the capacity is now only used to compare groups together and no more with the 1024 value > > I was hoping that we could decouple a full audit of the load-balance > code from this relatively simple patch set by staying with 1 for now. I > worry that the changing SMT capacity can turn into a major task. Just > proving that there is no regressions even if we know it should be, is a > lot of work. Yes, you are probably right on that point > > I'm happy to look at the SMT stuff it has been on my list of outstanding > issues for a very long time, but I would prefer to break it into > multiple independent patch sets to keep them focused. I haven't had a > much luck with massive complicated patch sets so far ;-)